1. Field of the Invention
The present invention relates to an acceleration sensor for detecting acceleration, which is used for toys, automobiles, aircrafts, portable terminals and the like, and particularly to an acceleration sensor that can be produced using a semiconductor technology.
2. Description of the Related Art
Acceleration sensors utilizing a change in physical quantity such as a piezo resistance effect and a change in electrostatic capacity have been developed and commercialized. These acceleration sensors can be widely used in various fields, but recently, such small-sized acceleration sensors as can detect the acceleration in multi-axial directions at one time with high sensitivity are demanded.
Since silicon single crystal becomes an ideal elastic body due to the extreme paucity of lattice defect and since a semiconductor process technology can be applied for it without large modification, much attention is paid to a piezo resistance effect type semiconductor acceleration sensor in which a thin elastic support portion is provided at a silicon single crystal substrate, and the stress applied to the thin elastic support portion is converted into an electric signal by a strain gauge, for example, a piezo resistance effect element, to be an output.
As a three-dimensional acceleration sensor, an acceleration sensor has been used, which comprises elastic support arms each of a beam structure formed by a thin portion of a silicon single crystal substrate connecting a mass portion constituted by a thick portion of a silicon single crystal substrate in a center and a frame in its periphery. A plurality of strain gauges are formed in each axial direction on the elastic support arms. In order to sense a small acceleration with an enhanced sensitivity, the elastic support arms are made long and/or thin, or the mass portion that works as a pendulum is made heavy. The acceleration sensor that can detect a small acceleration has led to an excessive amplitude of the mass portion, when subjected to a large impact, and resulted to break the elastic support arms. To avoid the break of the elastic support arms even if a massive impact is applied, regulation plates have been installed above and below the acceleration sensor element to restrict amplitude of the mass portion within a certain range.
An acceleration sensor having regulation plates is described in Japanese Laid-Open Patents HEI 4-274005, HEI 5-41148 and HEI 8-233851.
Japanese Laid-Open Patents HEI 4-274005 and HEI 8-233851 also disclose a method in which, to control a gap at a predetermined value between the regulation plates and the mass portion of the acceleration sensor element, small balls having a diameter of substantially the same distance as a gap are mixed with adhesive, and the adhesive with small balls mixed is used to bond regulation plates to the acceleration sensor element. The gap can be maintained at a predetermined value because the gap between regulation plates and the acceleration sensor element can be dictated by a diameter of small balls. The use of adhesive containing small balls thus enables the control of a gap between regulation plates and the acceleration sensor element.
Although such an excessive amplitude of the mass portion caused by a large acceleration can be prevented by the regulation plates mounted over/below the acceleration sensor as described in the Japanese patent documents, the elastic support arms may be broken when subjected to such a large impact. Especially an acceleration sensor for measuring as a small acceleration as less than 2 G has thin and narrow elastic support arms having a small cross sectional area and a long arm, while its mass portion is made heavy, in order to ensure to have an enhanced sensitivity. So, the elastic support arms may often be broken when an impact is applied. To prevent such breakage, the elastic support arms may be made thick but it will cause to reduce the sensitivity of the acceleration sensor.